Apparatus for the growth of single crystals by the Czochralski method includes a crucible to contain a melt in a mechanism for concurrently drawing a crystal from the melt on a vertical axis at a substantially steady rate while providing relative rotation about this axis for the growing crystal in respect to the melt. Vertical lift of the growing crystal is effectuated from the "pulling head" portion of the apparatus which must maintain mechanical alignment of the axis of rotation with the vertical pulling axis while providing ease of access for maintenance and rapid turn around time in a production context. Moreover, in one class of apparatus it is desirable to carry out the process in a vacuum-tight housing in order to obtain improved freedom from contamination.
It is known in prior art to employ a ball chain for supporting and vertically lifting the crystal as it is drawn from the melt together with a take-up drum for storage of the untensioned chain. In a known apparatus the alignment of the pulling axis, gravitationally defined by the ball chain with the rotation axis is maintained by causing the take-up drum to travel along a lead screw, transverse to the crystal pulling rotation axis. The known drum translation apparatus employs a keyed or splined mechanism or linear ball bushing to constrain the drum translation to its rotational axis during rotation of the drum. A fixed nut and lead screw on the axis of the drum have such a thread pitch to translate the drum by an amount equal to the width of the chain. Thus the chain is maintained tangent with the drum circumference and in constant alignment with the desired crystal rotation axis. The lubrication requirements for the lead screw and spline of the known chain pulling mechanism is clearly ill-suited to contain a contamination-free environment desired for crystal growth with the present invention.
The transverse travel requirements for the drum of this prior art mechanism further enlarges the volume of the pulling head and effects the balance, the mechanism which controls the rotation about the vertical axis. The winding on the chain of the drum is a source of uneven lift as the chain increments relax against the drum surface while under tension. Such relaxation occurs in an uncontrolled fashion and small irregularities in vertical pulling are thereby introduced.
Crystal pulling apparatus of the form above-described utilizes a "ball chain" taking the form of a linkage of spherical beads. In common with the prior art apparatus above-described, copending application Ser. No. 119,913 describes a compact pulling head which effectuates long pull lengths utilizing the ball-chain and a sprocket adapted to conform to the shape of the chain bead by formation of pockets in the lateral surface of the sprocket. The desired support and rotation axis is congruent with a tangent from the sprocket and untensioned chain is accommodated in a storage space proximate the sprocket. In this apparatus stabiity and control of the growing crystal are substantially improved while maintaining uniform rotational and vertical motions. This arrangement has a number of advantageous properties but in common with the afore-mentioned prior art mechanism, the chain or cable must be replaced at moderately frequent intervals of use. In the prior art apparatus the chain is secured to the drum directly by mechanical fasteners. Consequently, the pulling head must be opened to secure the fixed end of the replacement chain to the drum. In the apparatus of the copending application it is also necessary to open the pulling head to install a new chain although no drum must be aligned, nor must the chain be secured by a fastener to a take-up mechanism. Thus, both of the above described apparatus require disturbing large vacuum seals and further result in downtime of the order of hours.